An epoxy resin has excellent mechanical properties and thermal properties, and is therefore widely used in a variety of fields. An imidazole is used as a curing agent for curing such an epoxy resin, but the curing of an epoxy resin-imidazole mixed liquid starts quickly, and thus a problem thereof is that one-pack stability is extremely poor.
Then, as a curing agent, the use of an acid addition salt of an imidazole obtained by adding a hydroxybenzoic acid to an imidazole (see Patent Document 1), and the use of a clathrate of a tetrakisphenol type compound (such as 1,1,2,2,-tetrakis(4-hydroxyphenyl)ethane (hereinafter, referred to as TEP)) and an imidazole (see Patent Document 2) have been proposed. The acid addition salt of an imidazole and the clathrate exert a certain effect, but it has been desired to develop a curing agent having the same function or the enhanced function as those of the acid addition salt and the clathrate.
Under such circumstances, the applicant has developed a clathrate of an isophthalic acid type compound and an imidazole type compound, but the clathrate has been insufficient in terms of hardness in fields where higher hardness is required (Patent Document 3).